by Juan Manuel Vuletich
Also called Scripting with Players, this style is a descendent of the Self programming style. Morphs (i.e. instances of classes from the Morph hierarchy) are visually assembled. Smalltalk does not allow adding methods to instances, so other objects (called "Players") are added to hold the scripts.
"In the User-Scripting style, you construct surface graphics by directly assembling standard Morphic parts -- e.g. Rectangles, Images, Joysticks, etc., by dragging them from a Parts Bin and arranging them as desired, and then you add user-defined state and behavior by adding instance variables and writing methods for "Players" who represent the individual morphs you wish to script.
The user thus does not directly subclass any particular kind of Morph, but rather she assembles Morphs and gives them special state and behavior by associating them with "Players", which are the fundamental user-scriptable object types for User Scripting."
It is described in detail (including a tutorial) by Squeak Central in "Programming in Morphic".